Prior art workers have devised many types of pneumatic fastener driving tools for driving staples, headed and headless nails, clamp nails and the like. A pneumatic fastener driving tool of the type contemplated by the present invention generally comprises a body connected to a source of air under pressure. The body contains a cylinder provided with a piston/driver assembly. The cylinder is surmounted by a main valve. The main valve is shifted between a closed position engaging the upper end of the cylinder and shutting the cylinder off from the supply of air under pressure within the tool body, and an open position opening the upper end of the cylinder to air under pressure to actuate the tool.
Most prior art pneumatic fastener driving tools are intended to be connected to a source of fluid under pressure (generally air), at a certain level of supply pressure. This, in conjunction with the tool design configuration and parameters, determines the power generated by the tool. Thus, most prior art tools of this type generate a predetermined amount of power which is not adjustable. It is normally sufficient for the largest fastener to be driven by the tool, such power being the maximum amount generated by the tool.
Prior art workers have recognized, however, that under some circumstances it would be advantageous to be able to adjust the power generated by a pneumatic fastener driving tool, depending upon the nature of the fastener, the fastener size, the nature of the workpiece into which the fastener is to be driven, and the desired depth of the fastener in the workpiece. In instances where power adjustment was required or desired, the most common prior art approach was to provide an air pressure regulator in the line to the source of air under pressure. An air pressure regulator has the net effect of changing tool output power. Unfortunately, by virtue of its weight and complexity, an air pressure regulator cannot conveniently be designed as part of the tool, or be located close to the tool.
Another approach for power regulation is taught in U.S. Pat. No. 4,523,646. This patent is directed to a vented pneumatic fastener driving tool. In such a tool, air in the cylinder beneath the piston/driver assembly is vented to atmosphere during the drive stroke. This patent teaches the provision of a choke for regulating the flow of air beneath the piston/driver assembly to atmosphere during a drive stroke. This approach has certain drawbacks. In essence, this approach controls the resistance to the drive stroke. Since the resistance is a fraction of the power generated, this approach does not give a wide variation of power regulation. Furthermore, this approach is restricted to use on the vented type of pneumatic fastener driving tool.
In a pneumatic fastener driving tool, the size of the opening between the cylinder and the main valve is a very critical parameter in generating tool power. Reducing the opening results in less power for a given tool design configuration and for a given level of supply pressure. Enlarging the opening increases the power to its maximum level. The highest power level is limited by the well established principle of critical pressure ratio or sonic velocity through an orifice. The present invention is based upon the discovery that if the opening between the cylinder and the main valve, when the main valve is in its open position, can be adjusted by the tool operator by means accessible on the tool, a control of the amount of power output can be achieved. The means for accomplishing this can be built into the tool and will add only minimally to the weight of the tool depending on the regulator design and the materials used. The cost of the regulator of the present invention is a fraction of the cost of a good air pressure regulator. Furthermore, the regulator comprises a part of the tool and is readily available to the operator at any time. The amount of power can be adjusted by simply rotating a knob located on the top of the tool. Rotating the knob in one direction increases the tool power. Rotating the knob in the opposite direction decreases the tool power. Therefore, a power adjustment can be readily made by the operator at any time, depending upon the nature of the fasteners, the size of the fasteners, the nature of the workpiece, the depth to which the fastener is to be driven in the workpiece, and the like. The power regulator of the present invention is applicable to vented and non-vented tools.